bt.talib.CORREL not working properly

For the indicator bt.talib.CORREL, if I pass in two indicators with missing values in the middle, no correlation will be calculated after the first missing value, even the values are there afterward, as a demonstration(a and b are both line object, i only show them as array here):

In my actual case, both a and b are about 300 days long, and the missing values is about 30 days long in the middle of one data, and I tried to calculate correlation for a 6 days window, not sure if this has to do with issue. All the value before missing ones are correct.